The waste gypsum takes a gypsum dehydrate form and is generally named chemical gypsum and about 400,000 tons of waste gypsum annually is discharged from industries in the country which use sulfuric acid or generate sulfuric acid as wastes. The reuse of gypsum relies on a purity of gypsum and the gypsum having at least 94% of purity may be currently used in gypsum board, plaster, and the like, but the chemical gypsum which is currently produced has already exceeded a demand of gypsum industry. The fuel gas desulfurization gypsum which is discharged from the coal-fired thermal power plant is byproducts of which about 80 to 90% may be sold, but as the coal-fired thermal power plant is continuously increased and most of the chemical gypsum generated from a fertilizer production company is piled up out in the open, the reuse rate thereof cannot but be reduced, which leads to environmental pollution. One of the methods for resolving the above problems is a recycling method for recovering and reusing the ammonium sulfate and calcite from the waste gypsum.
As a method for producing ammonium sulfate using gypsum and ammonia, there is a method called a Mersberg process which was first suggested at the beginning of 19th century. The process has been experimentally used in 1960s in the United Kingdom and India. Meanwhile, in the US, a process of reproducing ammonium sulfate during a process of producing an ammonium phosphate ((NH4)3PO4) fertilizer at the beginning of the 1960s has been tested. It was reported that a typical reaction condition is maintained at 70° C. for 5 hours and a conversion rate reaches about 95%. Recently, a technology of producing ammonium sulfate and calcite by reacting ammonium carbonate ((NH4)2CO3) with gypsum by United States Geological Survey (Chou et al., 2005) has been researched. However, in this research, since an excessive amount of ammonium is injected and an endothermic reaction is used, more reaction cost may spend than a current international price of ammonium sulfate, such that it may be difficult to secure economic efficiency. Further, an initial temperature of the reaction becomes 50 to 60° C. and the recovery rate is low as 83%. Unfortunately, the international price of ammonium sulfate suddenly increased by about 30%; this negates the Mersberg process to be performed with economic efficiency by using the general chemical reaction as in the above-mentioned conditions. However, since all the chemical companies use byproducts to produce the ammonium sulfate internationally distributed currently, a sudden increase in the price may be minimal compared to the overall price of the process.
In the country, research for producing calcite as principal products and producing ammonium sulfate as byproducts by using gypsum for a mineral carbonation reaction by Korea Institute of Geoscience And Mineral Resources in 2008 (Korean Patent Laid-Open Publication No. 10-2010-0008342, published on Jan. 25, 2010, entitled “Sequestration of Carbon Dioxide by the Waste Gypsum”) has been conducted.
Further, since (a) a method for separating livestock excretion into solid components and liquid components in mass, (b) a method for collecting a CO2 gas and an ammonia gas, (c) a method for reacting a separated liquid component with a collected CO2 gas and ammonia gas, and the like which are disclosed in Korean Patent No. 10-0723066 (Title of the Invention: Fertilizing Process Of Livestock Excretion And System Thereof) are not practical and concrete and a content of the used ammonia and CO2 is never mentioned in the above Patent, a ratio of the produced calcite and ammonium sulfate may not be confirmed and efficiency is very low, such that the possibility to recycle the produced ammonium sulfate or the possibility to secure economic efficiency of the produced ammonium sulfate is extremely small actually.
Further, “Production of Ammonium Sulfate Using Phosphor-gypsum” (Seoul University, 1983) by Shin Yoon Kyeung, which is known prior to this application, describes a method for producing ammonium sulfate by using ammonium carbonate and gypsum as raw materials, but the reaction is a two step reaction in which (a) ammonium carbonate is first produced and (b) the produced ammonium carbonate is reacted with gypsum, and therefore the process is complicated and the reaction of ammonium carbonate with gypsum is an endothermic reaction and therefore requires heat (see the following Reaction Formula 1). Further, the method proposed by Shin Yoon Kyeung does not mention the production efficiency of ammonium sulfate and calcite and uses a stoichiometric composition to be slightly far from recycling.NH3+H2O+CO2→(NH4)2CO3 (NH4)2CO3+CaSO4.2H2O→CaCO3+(NH4)2SO4+12KJ (endothermic reaction)  Reaction Formula 1
Further, if gypsum, ammonia, and CO2 are mixed with one another at a stoichiometric ratio and are reacted with one another, when considering the price and the reaction cost and the reaction efficiency of the raw materials, the economic efficiency may not be secured, and therefore the process cannot but be limited to a scientific meaning. For example, if it is assumed that 100,000 tons/year of gypsum is processed, a loss from about 20 billion won to over about 50 billion won is anticipated in computation.
As described above, the method of producing ammonium sulfate by using gypsum has been suggested and attempted from a long time ago. However, when starting materials are not injected at a specific ratio so as to produce an ammonium sulfate fertilizer by using the gypsum, a purity of calcite and ammonium sulfate which are post-reaction products is reduced and the reaction efficiency and recovery rate are reduced, such that production costs may be increased. However, the ammonium sulfate currently distributed at home and abroad is produced by using industry byproducts without exception and therefore is much cheaper than a commonsensical price. That is, since the ammonium sulfate produced by using the gypsum is absolutely disadvantageous in the contention with the ammonium sulfate which is being currently distributed, the ammonium sulfate produced by using the gypsum may not be recycled and the process of converting the gypsum into the ammonium sulfate may be meaningless.